This invention relates to color printing, and more particularly to a method for automatically adjusting color correction. In addition, this method may be applied to white balance adjusting apparatus such as that used in a color video camera.
In photographic printing, it is a well known practice to correct the color balance of a print from an original, such as a color photographic negative, by adjusting the printing process so that the integral density of the resulting print is neutral or grey. This correction strategy is based on the assumption that the overall average color of a scene integrates to a grey color. This strategy is very effective at reducing the effects resulting from scene illuminants that are spectrally different such as tungsten and daylight. In a like manner, image sensing apparatus such as a video camera, average typically, over a relatively long time period, color difference signals, R-Y and B-Y, to a zero value. This is equivalent to integrating to grey.
These methods work well for the majority of scene and illuminant combinations. However, when the scene subject matter is highly colored, particularly with a single dominant color, the integrate to grey strategy fails as this dominant scene color is mistaken for an illuminant bias. This failure, known as subject failure, produces unpleasant color casts in the color complimentary to the dominant scene color. There are various strategies for minimizing these failures. These strategies are typically based on reducing the amount of correction based on a population of images and/or on information in neighboring frames. The Agfa MSP printer is an example where information in neighboring frames is used to improve color correction.
In addition, it is possible, particularly with digitization, to extract additional information from an image that can be used to improve color correction and reduce the number of subject failures. Many of these methods, such as that taught in U.S. Pat. No. 5,555,022, divide the scene information into a plurality of regions representing different locations within a scene. Means to select and weight the correction of these regions are then employed to provide automatic white balancing in a video camera. In addition, restricting the degree to which color correction gain is applied is taught.
Another approach that combines color correction with tone scale corrections is based on random sampling within a digitized image and subsequently modifying the resulting histogram of these samples. U.S. Pat. No. 4,677,465 issued Jun. 30, 1987, to J. S. Alkofer, entitled xe2x80x9cDigital Color Image Processing Method with Shape Correction of Histograms Used to Produce Color Reproduction Functions,xe2x80x9d and U.S. Pat. No. 4,729,016 issued Mar. 1, 1988 to J. S. Alkofer, entitled xe2x80x9cDigital Color Image Processing Method and Apparatus Employing Three Color Reproduction Functions for Adjusting Both Tone Scale and Color Balance,xe2x80x9d disclose relatively complex methods that utilize these samples in a plurality of segmented contrast intervals through normalization techniques and with comparison to image population data.
It is an object of the present invention to provide an improved color image by using an image processing method and means that overcome the problems, such as color subject failure and dependency on film population data, noted above.
The method of the present invention is based on the assumption that the red, green and blue standard deviations, particularly of high spatial frequency information as calculated in a particular space, e.g. a power law space, should be equal.
According to the present invention there is provided a method of correcting the color balance of a color image comprising a plurality of channels, comprising the steps of:
capturing the image;
transforming each channel of the image into a power law space;
storing the transformed channels;
spatially filtering the transformed channels with at least one filter to generate at least one filtered image;
calculating the standard deviation of each of the filtered transformed channels;
determining a reference standard deviation;
forming a ratio between the reference standard deviation and the standard deviation of each filtered transformed channel; and
applying the ratio as a multiplicative gain factor to each stored transformed channel to provide a color corrected image.
The channels may be filtered spatially with either band-pass filters or high-pass filters. It is possible to include additional steps to further improve the color of the image.
The invention further provides means for correcting the color balance of an image comprising a plurality of channels, comprising:
means for capturing the image;
means for transforming each channel of the image into a power law space;
means for storing the transformed channels;
filter means for spatially filtering the transformed channels to generate at least one filtered image;
calculating means for calculating the standard deviation of each of the filtered transformed channels;
means for determining a reference standard deviation;
means for calculating the ratio between the reference standard deviation and the standard deviation of each of the filtered transformed channels; and
means for applying the ratio as a multiplicative gain factor to each of the stored transformed channels to provide a color corrected image.
Advantageous Effect of the Invention
The present invention provides a color image with significantly reduced color biases or correction errors than those provided by methods and apparatus found in the prior art.
The above and other objects, features and advantages of the present invention will become apparent from the following description of a preferred embodiment, in connection with the accompanying drawings.